1. Field of the Invention
The present invention relates to an ink droplet ejection device and an ink droplet ejection method.
2. Discussion of Related Art
As an ink droplet ejection device, there is known a recording head that is to be incorporated in an inkjet printer. U.S. Pat. No. 6,663,208 (corresponding to JP-2002-160362A) discloses such a recording head including: (a) a cavity unit having (a-1) a plurality of nozzles located in its front portion and (a-2) a plurality of pressure chambers located in its rear portion and held in communication with the respective nozzles; and (b) a piezoelectric actuator unit fixedly disposed on the rear portion of the cavity unit. The piezoelectric actuator unit includes a plurality of deformable portions serving as actuators. Each of the deformable portions is arranged to be deformable with application of a drive pulse signal (voltage) thereto so as to change a volume of a corresponding one of the pressure chambers and apply an ejection pressure to an ink stored in the corresponding pressure chamber, so that the ink is ejected from the corresponding pressure chamber through one of the nozzles that is held in communication with the corresponding pressure chamber. The ejected ink takes a form of an ink droplet that is received in a recording medium, whereby an ink dot is formed on the recording medium. The recording head is arranged to be reciprocably movable in a main scanning direction (i.e., a width direction of the recording medium) that is perpendicular to a sub-scanning direction (i.e., a feeding direction of the recording medium).
It is common that an inkjet printer has a plurality of recording modes which are different with respect to size and density of the ink dot that is to be formed on the recording medium. Depending upon a selected one of the recording modes, the movement velocity of the recording head and the number of ink droplet or droplets constituting the ink dot are changed.
As discussed in the above-identified U.S. Pat. No. 6,663,208, upon ejection of the ink droplets, there is a case where extra droplets called “satellite” are ejected in addition to main droplets. The satellite droplets could be caused, for example, where a plurality of ink droplets are successively ejected so as to cooperate with each other to form the ink dot. The successive ejections of the plurality of ink droplets are made by pressure fluctuation caused in the corresponding pressure chamber. Such pressure fluctuation, in general, cannot be sufficiently terminated upon completion of the successive ejection, so that extra droplets are inevitably ejected as the satellite droplets, due to the pressure fluctuation remaining in the corresponding pressure chamber even after the completion of the successive ejection. That is, the satellite droplets are caused principally by the residual pressure fluctuation. If the satellite droplets are received by the recording medium, it is not possible to obtain a recorded image as desired, resulting in deterioration in the recording quality.
The satellite droplets are not likely to be caused in an operation with a recording mode for a high image resolution in which a recording operation is performed by small-sized dots each formed of a small-sized ink droplet with the recording head being moved at a relatively low speed. On the other hand, the satellite droplets are likely to be caused in an operation with a recording mode for a low image resolution in which a recording operation is performed by large-sized dots each formed of a plurality of ink droplets for reducing a length of time to complete the recording over a certain unit of area. That is, for forming each dot with a plurality of ink droplets, a plurality of successive drive pulses are applied to the corresponding actuator in a short length of time. Pressure waves generated by the successive drive pulses remain in the ink stored in the corresponding pressure chamber even after the ink ejection performed by a final one of the successive drive pulses, and the remaining pressure waves cause undesirable ejection of the satellite droplets.
In view of the above problem, the present inventor has studied to obtain a waveform of pulse train effective to avoid occurrence of the satellite droplets even in the low resolution mode in which each ink dot is required to have a large size. The study was conducted with assumption that the recording operation is to be performed with a recording mode for a low image resolution, specifically, with an image resolution of about 600 dpi×600 dpi (dot per inch). In this low resolution mode, it is considered, by taking account of a volume of each one droplet in a standard resolution mode, that each one dot requires to be constituted by about three droplets.
In an inkjet head, commonly, an ejection velocity of each ink droplet is changed depending upon a pulse width of the drive pulse. The relationship between the ejection velocity and the pulse width is represented by a curved line that is convex upward, as shown in a graph of FIG. 10A. In this graph, where the pulse width of the drive pulse is set at a value T0, the ejection velocity is peaked or maximized. Since the ink droplet is ejected most efficiently with the pulse width being set at the value T0, not only the ejection velocity but also a volume of the ejected ink droplet is peaked or maximized at the value T0.
The present inventor has tested a drive pulse train, as shown in FIG. 10B, including (a) three successive drive pulses each having a pulse width that is set at the above-described maximizing value T0, for causing successive ejection of three ink droplets (i.e., ejection of the ink of amount that is three times as large as an ordinary amount) and (b) one complementary drive pulse (canceling signal) for canceling residual pressure waves that have been generated by the preceding three successive drive pulses. The complementary drive pulse has a pulse width that is small so as not to cause ejection of another ink droplet. The pulse widths of the three successive drive pulses and the complementary drive pulse are respectively set to cooperate to satisfy the following expressions:T1=T2=T3=T0,
T4<<T0, where “T0” represents the maximizing value; “T1” represents the pulse width of a first one P1 of the three successive drive pulses; “T2” represents the pulse width of a second one P2 of the three successive drive pulses; “T3” represents the pulse width of a third one P3 of the three successive drive pulses; and “T4” represents the pulse width of the complementary drive pulse P4 following the three successive drive pulses.
The test revealed that the ejection of the satellite droplets can not be satisfactorily prevented in the above-described drive pulse train (as shown in FIG. 10B). That is, the ejection of the satellite droplets can not be sufficiently prevented only by the complementary drive pulse P4 provided after the three successive drive pulses P1, P2, P3 and serving as the canceling signal. It might be possible to satisfactorily cancel the residual pressure waves, for example, by adding another complementary drive pulse serving as another canceling signal in the drive pulse train. However, the increase in the number of the drive pulses in a drive cycle for covering one dot affects a drive cycle for covering the subsequent dot. In this sense, the increase in the number of the drive pulses is not feasible.
Further, there is another problem originating from difficulty in equally manufacturing a large number of recording heads without variation among the individual recording heads. That is, even among the recording heads of the same specification, there could be some difference in characteristic or performance for ink ejection. The recording heads could be different from one another with respect to the relationship, too, which is represented by the curved line in the graph of FIG. 10A. Consequently, the volume of the ejected ink droplet at the above-described maximizing vale T0 is not necessarily the same in all of the recording heads, but could vary from one to another. Particularly, where a recording operation is performed by large-sized dots each formed of a plurality of (e.g., three) ink droplets ejected with the pulse width of the maximizing vale T0, as described above, the variation in volume between the ink droplets leads to a variation in size between the dots. The variation in size of the dots is a plurality of times (e.g., three times) as large as the variation in volume between the ink droplets, and accordingly is too large to ignore.